linux-kernel - Re: [PATCH v2 0/4] /proc/stat: Reduce irqs counting performance overhead

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Message-ID: <20190109185226.GP6310@bombadil.infradead.org>
Date:   Wed, 9 Jan 2019 10:52:26 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Miklos Szeredi <miklos@...redi.hu>,
        Daniel Colascione <dancol@...gle.com>,
        Dave Chinner <david@...morbit.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH v2 0/4] /proc/stat: Reduce irqs counting performance
 overhead

On Wed, Jan 09, 2019 at 01:37:23PM -0500, Waiman Long wrote:
> On 01/09/2019 01:24 PM, Matthew Wilcox wrote:
> > Did you even try just using the general purpose infrastructure that's
> > in place?  If that shows a performance problem _then_ it's time to make
> > this special snowflake just a little more special.  Not before.
> 
> I have looked into the percpu counter code. There are two aspects that I
> don't like to introduce to the interrupt handler's code path for
> updating the counts.
> 
> 1) There is a raw spinlock in the percpu_counter structure that may need
> to be acquired in the update path. This can be a performance drag
> especially if lockdep is enabled.
> 
> 2) The percpu_counter structure is 40 bytes in size on 64-bit systems
> compared with just 8 bytes for the percpu count pointer and an
> additional 4 bytes that I introduced in patch 2. With thousands of irq
> descriptors, it can consume quite a lot more memory. Memory consumption
> was a point that you brought up in one of your previous mails.

Then _argue that_.  Don't just go off and do something random without
explaining to the rest of us why we're wrong.